#8827: Inferring Safe mode with GeneralizedNewtypeDeriving is wrong -------------------------------------+------------------------------------ Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.1 Component: Compiler | Version: 7.8.1-rc2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: 8226, 8745 -------------------------------------+------------------------------------ Comment (by nomeata):
I'll admit to liking the general flavor of comment:27, but the details elude me somewhat. What, precisely, would go after the word deriving?
Well, either `data MyList a = [a] deriving Coercible`, which will give you the same instance as described in the paper, but independent of the scope of the constructor. Or `deriving instance Coercible a b => Coercible (Map k a) (Map k b)`, which allows you to control the coercing This is even strictly more powerful that our current system: It would allow the author of `Map` to export the instance above, while still using `coerce` in his code (with constructors in scope and no abstraction to follow) to coerce the key, if he wishes to do so.
What's the interaction between derived instances and role annotations? Could a derived instance be more restrictive than the assigned roles? (It certainly couldn't be less restrictive!) How would a user specify that?
As you say: The derived instance have to adhere the roles, but may be more restrictive (if created using a standalone deriving instance). The non- standalone-deriving instance would be the one that we have currently. OTOH, this gets hairy once we mix tycons with exported constructors and tycons without, but with exported Coercible instances, in a new data declaration and try to find out when we can coerce under ''that''... and that issue is well handled by roles and role inference. *shrug* -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8827#comment:30 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler